1. Field of the Invention
The present invention relates to black sintered bodies of aluminum nitride used as materials for insulating substrates, heat sinks, packages for semiconductor devices and a process for producing the same. More specifically, the present invention relates to sintered bodies of aluminum nitride having a high density and a good thermal conductivity and to a process for the production thereof at reduced sintering temperatures.
2. Description of the Prior Art
In recent years, with an increasing trend toward the high integration and high densification of LSI, there has been a great demand for insulating substrate materials having a good thermal conductivity. Heretofore, alumina sintered articles have been most widely used as a material for an insulating substrate. However, since the alumina substrate has a low thermal conductivity (about 20 W/m.K) and has a large thermal expansion coefficient as compared to silicon, it has many problems such as poor bondability, etc. Therefore, sintered bodies of aluminum nitride (AlN) which have a good thermal conductivity and a desirable thermal expansion coefficient approaching that of silicon have attracted a great interest. In addition to these advantages, the aluminum nitride sintered bodies possess a high mechanical strength (40-50 kg/mm.sup.2) and good electrical properties, for example, a small dielectric constant.
However, since aluminum nitride materials are poor in sinterability due to their typically covalent bond, it is difficult to obtain high density sintered bodies. For this reason, various ways of achieving high density sintered bodies of aluminum nitride have been tried to date and, for example, there may be mentioned the addition of a sintering aid and the use of hot pressing. As the sintering aid, a rare earth oxide or alkali earth oxide is added to aluminum nitride powder and the resulting mixture is sintered in a nonoxidizing atmosphere at atmospheric pressure or by using a hot pressing process. The resulting sintered bodies of aluminum nitride exhibit a relatively high density but are insufficient in thermal conductivity. Further, the known aluminum nitride composition requires a very high sintering temperature, i.e., at least 1800.degree. C., and results in an increased production cost.
On the other hand, there has been recently obtained highly pure aluminum nitride powder. As an example, in Japanese patent application laid-open No. 60-71 575, transparent sintered bodies of aluminum nitride are produced at atmospheric pressure or using a hot pressing process. However, in the production of such transparent sintered bodies, uneven sintering and uneven coloring problems are apt to arise, and, in some cases, color change occurs in the resulting sintered bodies, for example, by sunlight (ultraviolet rays). When such sintered bodies are used as a packaging substrate, an IC memory mounted inside will be detrimentally affected.